An analog digital converter (ADC) is a circuit which converts an analog input signal into a digital output signal. In order to raise the sampling frequency, a time-interleaved ADC has been proposed in which a plurality of ADCs (ADC channels) are provided and this plurality of ADCs convert an analog input signal into a digital output signal, successively by time division. A time-interleaved ADC is capable of high-speed operation, but the S/N ratio may decline if the characteristics of each ADC are different and/or there is deviation in the relationship between the timings at which the ADCs operate.
Foreground calibration and background calibration have been proposed as methods for correcting error between ADC channels. The former requires time for correction apart from the normal ADC operating time. The latter, on the other hand, performs correction during normal operation of the ADC, and therefore also corrects errors caused by temporal change or temperature variation, and the like, in the background.
S. M. Jamal, et. al., “A 10b 120Msample/s Time-Interleaved Analog-to-Digital Converter With Digital Background Calibration”, JSSC 2002, describes an ADC which performs background calibration.
According to the background calibration circuit described above, an adaptive filter is provided in at least one of a plurality of ADC channels which operated by time-interleaving (time division), and the coefficient value of the adaptive filter is calculated on the basis of the sum output obtained by summing the outputs of the plurality of ADC channels. By controlling the coefficient value of the adaptive filter so as to suppress the spurious signal component of the summed output, it is sought to suppress the spurious signal component (error signal component, image signal component) which is error generated as a result of deviation (skew) in the sampling timings of the time interleaving method.
However, in the method described in S. M. Jamal, et. al., “A 10b 120Msample/s Time-Interleaved Analog-to-Digital Converter With Digital Background Calibration”, JSSC 2002, it is not possible to suppress the spurious signal component adequately when the input signal is of a specific frequency, and this method can only be used for analog input signals of limited frequencies.